Pulsed laser deposited CoFe2O4 thin films as supercapacitor electrodes

The influence of the substrate temperature on pulsed laser deposited (PLD) CoFe2O4 thin films for supercapacitor electrodes was thoroughly investigated. X-ray diffractometry and Raman spectroscopic analyses confirmed the formation of CoFe2O4 phase for films deposited at a substrate temperature of 450 °C. Topography and surface smoothness was measured using atomic force microscopy. We observed that the films deposited at room temperature showed improved electrochemical performance and supercapacitive properties compared to those of films deposited at 450 °C. Specific capacitances of about 777.4 F g−1 and 258.5 F g−1 were obtained for electrodes deposited at RT and 450 °C, respectively, at 0.5 mA cm−2 current density. The CoFe2O4 films deposited at room temperature exhibited an excellent power density (3277 W kg−1) and energy density (17 W h kg−1). Using electrochemical impedance spectroscopy, the series resistance and charge transfer resistance were found to be 1.1 Ω and 1.5 Ω, respectively. The cyclic stability was increased up to 125% after 1500 cycles due to the increasing electroactive surface of CoFe2O4 along with the fast electron and ion transport at the surface.


Introduction
With the endless desire for electricity and dri from the conventional power grid to renewable energy sources, the demand for efficient energy storage devices is rising exponentially. For this, the supercapacitor (SC) is an excellent solution due to its fast charging/discharging rates, high power density, long cycle life, and eco-friendly nature. [1][2][3][4] In general, SCs can be classied into two types based on the operational chargedischarge storage mechanism: electric double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs work on the principle of electrostatics, where the energy is stored by trapping ions at the electrode/electrolyte interface. 2 A pseudocapacitor is based on the faradaic charge transfer process that utilizes reversible redox reactions and intercalation at electrodes to store charge. In particular, pseudocapacitive materials can achieve excellent specic capacitances and energy densities, and they can thus be an interesting source for constructing novel energy storage devices. 5 However, the pseudo capacitor has a comparatively shorter cycle life than an EDLC, due to the not completely reversible redox reactions and the loss of electrical contact resulting from the disintegration of the crystal structure, which reduces electrochemical performance. This can be overcome by combining a pseudo-active material a with conductive support, also known as hybrid supercapacitors. 6 Transition metal oxides, such as RuO 2 , Co 3 O 4 , and Fe 2 O 3 have been considered as active materials for supercapacitors on account of their high theoretical capacitance, variable oxidation states, environmentally friendly nature, and low cost. 7,8 Further, several researches have been focused to improve performance, storage, and energy densities for SCs by either tailoring the material properties 9,10 and/or altering the surface 11 of the electrode materials. Some of the prominent approaches include the electrode made of porous carbon, 12 carbon nanotube, 13 and metal-organic frameworks. [14][15][16] Recently, mixed metal oxides with spinel ferrite structure, MFe 2 O 4 (M ¼ Co, Mn, Zn, Mg, or Ni), were reported to have distinct hard or so magnetic properties or even superparamagnetism, a large range of oxidation states, and chemical stability. 17 In spinel ferrites, the divalent metal ion (M 2+ ) occupies the tetrahedral site, and the trivalent metal ion (M 3+ or Fe 3+ ) occupies the octahedral position of the cubic close-packed oxygen lattice, see Fig. 1 20 These properties of ferrites were explored by several researchers who showed that the ferrite electrode has superior pseudo-capacitance. [21][22][23] It is anticipated that mixed ferrites, with varying ratios of the M and Fe ions, improve the electrode performance, due to their higher number of cations for coordination sites. 24 Among the known mixed spinel ferrites, CoFe 2 O 4 has a variety of merits, including excellent chemical stability, efficient electrocatalytic behaviour, high specic capacitance that makes it a suitable candidate for electrodes in supercapacitors. [25][26][27] To harness these advantages of CoFe 2 O 4 for supercapacitor applications, numerous chemical methods of preparation were reported, such as solvothermal, hydrothermal, electrodeposition, and spin coating. 17,[28][29][30] Even though all these chemical methods are cheap, they all have issues with the reproducibility of the stochiometric composition of the deposited material. On the other hand, the potential of the pulsed laser deposition (PLD) technique, which has an excellent stochiometric reproducibility of the target material, 31,32 has not yet been explored for the deposition of cobalt ferrite electrodes for supercapacitor application.
The present study reports the efficacy of PLD deposited CoFe 2 O 4 thin lms for supercapacitive electrode applications. The lms with different crystallinity were prepared by using different substrate temperatures. Mesoporous lms prepared at RT can provide a higher surface area for an electrolyte to diffuse with low transfer limitation. The low crystallinity of the electrode material has improves the exposure to active sites accessible for the electrolyte on the surface.

Experimental
Cobalt ferrite thin lms were deposited on a fused quartz substrate in a PLD vacuum chamber with a base pressure of 2 Â 10 À5 mbar. A sintered cobalt ferrite target comprised of a-Fe 2 O 3 and Co 3 O 4 in the stoichiometric ratio was repetitively (10 Hz) irradiated with the third harmonic (l ¼ 355 nm) of an Nd:YAG laser for 6 ns pulse duration delivering a typical uence of 2.5 J cm À2 on the target during deposition. The fused quartz substrate was kept at a distance of 45 mm from the target. In this way, we fabricated CoFe 2 O 4 thin lms at room temperature (RT), 350 C, and 450 C substrate temperature. A clear evidence of crystallisation in the X-ray diffraction studies was observed only at 450 C substrate temperature and the electrochemical performance of the 350 C lms (not shown here) was similar to that of the RT lms. Thus, for the ease of discussion, only the RT and 450 C samples are considered for the further investigations. Thicknesses of lms prepared at RT and 450 C substrate temperature were found to be around 110 nm and 200 nm, respectively.
The crystallographic investigation of the lms was performed by recording X-ray diffraction (XRD) 2q-u scans using a Bruker D2 PHASER X-ray diffractometer. Raman measurements were carried out using XploRA PLUS Raman spectrometer from Horiba scientic under 532 nm diode-pumped solidstate (DPSS) laser excitation using a 100Â/0.9 NA objective with a laser power density of 1.33 mW mm À2 in a back-scattering geometry. The collected Raman signals were dispersed onto an electron multiplier charge-coupled device (EMCCD) using a 1200 l mm À1 grating. The sample surface morphology was examined using an Agilent 5420 atomic force microscope operated in intermittent contact mode. The electrochemical measurements were carried out in the three-electrode system using Autolab's potentiostat PGSTAT302.
For electrochemical measurements, cobalt ferrite lms were used as both electrode material and current collectors. For a seamless electrical contact to the lm, a small piece of silver was clipped onto the conductive cobalt ferrite lm by a toothless alligator clip to connect the battery analyser. Then the assembly was dipped in the 1 M KOH aqueous electrolyte solution for electrochemical measurements. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out with conventional three-electrode arrangement comprising platinum as a counter electrode, a saturated calomel electrode (SCE) as a reference electrode, and CoFe 2 O 4 as a working electrode. Cyclic voltammetry measurements were carried out at various scan rates of 10-100 mV s À1 between the potential window of 0 V and 0.5 V. Galvanostatic charge-discharge measurements were carried out at a charging current density (0.5 mA cm À2 ) between 0 V and 0.5 V vs. SCE. The specic capacitance of the supercapacitor was calculated from GCD curves according to the equation 19 Here, C sp is the specic capacitance (F g À1 ), I is the response current density (mA cm À2 ), m is the mass of the electrode (g), DV is the potential range (V), and Dt is the discharging time.

Structural analysis
XRD 2q-u scans of the sample deposited at substrate temperatures of RT and 450 C are shown in Fig. 2. The lm deposited at RT does not show any XRD peak, implying that the lm is amorphous. However, the lm grown at a substrate temperature of 450 C shows sharp diffraction peaks and the peaks observed can be indexed to the cubic Co-ferrite phase with the help of However, the relative intensity of the CoO peaks with respect to the CoFe 2 O 4 peaks is minimal and the Co is thus expected not to inuence the electrochemical performance of the CoFe 2 O 4 electrode. We can also see in Fig. 2 that a lower number of peaks compared to the bulk were observed in the lm. The lms generally showed a preferred orientation in the (311) direction, as it is the case for many other reports of CoFe 2 O 4 thin lms. 7,33 The lattice constant for the lm deposited at 450 C calculated using the (511) peak was found to be 8.4Å which is comparable to the bulk crystallite value (ICDD, PDF 221086). For lms deposited at 450 C, the average crystallite size calculated by the Debye Scherrer formula using the (511) peak was found to be (19 AE 1) nm. The lm deposited at RT may have grains with very small size, which could not be detected by XRD. In our previous work, reporting CoFe 2 O 4 lm preparation at different temperatures by PLD, we observed similar XRD behaviour for lms prepared at RT. 34 The selected area electron diffraction (SAED) pattern of these lms prepared at RT showed diffraction rings which were identied as the Co-ferrite phase. This indicates that our lms deposited at RT consist of low crystalline CoFe 2 O 4 phase.

Raman spectroscopy
The Raman spectra of cobalt ferrite thin lms prepared at RT and 450 C are shown in Fig. 3. Raman spectroscopy is a powerful technique in understanding crystal structures of a material down to nano-size domains. CoFe 2 O 4 belongs to the space group Fd 3m, predicting 39 phonon modes for this spinel structure. Among them, the ve Raman active modes are A 1g , E g , and 3T 2g . 35,36 The modes below (above) 600 cm À1 are attributed to the oxygen motion around the octahedral (tetrahedral) lattice sites. 37 The broadening of the Raman modes indicating its amorphous nature is in agreement with the XRD results. The lm deposited at 450 C substrate temperature shows four prominent Raman active modes at 302 cm À1 , 458 cm À1 , 556 cm À1 , and 678 cm À1 , which correspond to E g , 2T 2g , and A 1g phonon vibrations, respectively. The Raman active T 2g mode around 200 cm À1 is absent in the spectra, probably due to the weak Raman cross-section of this particular mode. The A 1g Raman modes of the lm prepared at 450 C show a slightly asymmetric shape with a shoulder at the low-frequency end. This can be explained by the cationic radii at the octahedral and tetrahedral site (either Co or Fe) and the Co/Fe-O bond distance. Since Raman spectra are sensitive to the local structural change, the distribution of the local bond length can produce this double peak-like shape in the Raman spectra. 38 In the case of other modes, this asymmetry is not visible, which may be due to the relatively low Raman intensity. The presence of four well-resolved Raman modes in the Raman spectrum of the sample prepared at 450 C conrms the better crystalline quality compared to lms prepared at RT. The Raman results are in good agreement with the XRD results presented in the previous section.

Topographic analysis
For the topographic characterisation, atomic force microscopy (AFM) was performed at various regions on the CoFe 2 O 4 thin lms prepared at RT and 450 C. Three-dimensional (3D) micrographs of representative AFM images of both samples are   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 19353-19359 | 19355 shown in Fig. 4. The AFM micrograph analysis reveals that the sample prepared at RT has higher roughness and porosity compared to lms deposited at 450 C. Thus, the sample prepared at RT has a higher surface-to-volume ratio, which provides an easy path for ion movement at the electrode-electrolyte interface, making it suitable for supercapacitor applications. The surfaces of samples prepared at RT and 450 C have root mean square (RMS) roughness values of (20 AE 1) nm and (8.5 AE 0.5) nm, respectively.

Electrochemical studies
3.4.1. Cyclic voltammetry study. To study the supercapacitive behaviour of the pulsed laser deposited CoFe 2 O 4 thin lms prepared at different substrate temperatures, cyclic voltammograms (CV) in the alkaline medium were measured using 1 M KOH solution as the electrolyte. The effect of the scan rate on specic capacitances was investigated and Fig. 5(a) and (b) show the variation of voltammetric current at different scan rates for lms prepared at RT and 450 C, respectively. The electrochemical measurements of both types of CoFe 2 O 4 electrodes clearly show a highly pseudo-capacitive nature. The voltammetric currents are increase with the scan rate, and this behaviour is similar to that of an ideal capacitor. 39 The CV curves correspond to the conversion between different iron and cobalt oxidation states. The redox reaction of CoFe 2 O 4 was mainly ascribed to the redox pairs Co(III)/ Co(II), Co(IV)/Co(III), and Fe(VI)/Fe(III). 40,41 The superposition of the redox processes corresponding to the transitions of these redox pairs resulted in the broad CV peaks. 42 The electrode materials with intercalation pseudocapacitance can demonstrate nonnegligible CV peaks. 43,44 CV curves similar to our results were recently published and interpreted in terms of supercapacitor behaviour. 19,28,45 Both lms studied here showed good adherence to the substrate and these electrodes also did not crack aer electrochemical measurements. Fig. 5(c) shows CV recorded at 10 mV s À1 for both electrodes and it can be seen that the electrode prepared at RT shows larger charge storage capacity as compared to the electrode prepared at 450 C substrate temperature. The observed better charge storage capacity of RT prepared electrode might be due to favoured ion intercalation/deintercalation related to the poor crystallinity. The crystalline quality of of the 450 C electrode is better, as depicted in the XRD study, which restricts the ion intercalation/deintercalation.  3.4.2. Galvanostatic charge-discharge study. To understand the capacitive nature of the CoFe 2 O 4 electrodes, galvanostatic charge-discharge (GCD) measurements were carried out. Fig. 6 shows the charge-discharge curves of CoFe 2 O 4 electrodes prepared at RT and 450 C in a 1 M KOH solution at a current density of 0.5 mA cm À2 . As can be seen in the gure, the discharge curves for both types of electrodes are nonlinear. Such behaviour was already reported for CoFe 2 O 4 electrode materials used for supercapacitor applications. 28,45 According to Elkholy et al., such nonlinear discharge curves indicate that the capacitive performance is due to pseudocapacitance. 19 This can be attributed to the electrochemical adsorption-desorption reaction at the electrode-electrolyte interface. 46 The specic capacitance calculated according to eqn (1) from the GCD curves for CoFe 2 O 4 thin lms deposited at RT and 450 C was about 777.4 F g À1 and 258.5 F g À1 , respectively, at 0.5 mA cm À2 current density, where the mass loading of the CoFe 2 O 4 electrodes deposited at RT and 450 C is 0.0305 mg and 0.0419 mg, respectively. The higher capacitance of lms prepared at RT might be due to the higher surface area of the rough electrode material. Higher surface area could facilitate the electron transfer between the electrolyte and electrode and a greater wettability of the electrode during the charge/discharge process. In addition, amorphous materials can provide an easier pathway for the intercalation and deintercalation of charges, which improves the charge-transfer. 47 The energy density and power density of CoFe 2 O 4 lms prepared at room temperature were calculated by the equations 19 E:D: ¼ 0:5 Â C sp Â dV 2 3:6 (2) and P:D: where E.D. is energy density, P.D. is power density, C sp is specic capacitance, dV is a potential window of the discharge curve (here dV ¼ 0.4 V), and T d is the discharge time. The CoFe 2 O 4 lms deposited at room temperature exhibit an excellent power density (3277 W kg À1 ) and energy density (17 W h kg À1 ) compared to previously reported ferrite-based materials, as shown in Table 1. The values obtained for the power density lies in the supercapacitive regime in the Ragone plot shown in ref. 43. This further conrms the supercapacitive nature of the investigated electrodes. 3.4.3. Electrochemical impedance study. Electrochemical impedance spectroscopy (EIS) is an excellent tool to get information about series resistance and charge transfer resistance of a material. Fig. 7 shows the Nyquist plots of CoFe 2 O 4 thin lms prepared at RT and 450 C measured in 1 M KOH electrolyte. The spectrum is taken in the frequency range from 1 Hz to 10 kHz. In the high-frequency region, the intercept with the real part (Z 0 ) represents the combined solution resistance (R s ), which contains the ionic resistance of the electrolyte, the intrinsic resistance of active material, and the contact resistance. 28 The semicircle shown in the inset represents the charge-transfer process at the electrode-electrolyte interface. Both types of samples show low series resistance (zR s ¼ 1.1 U) and charge transfer resistance (zR ct ¼ 1.5 U). In the low-frequency region, a straight-line curve close to 45 with respect to the Z 0 axis for both types of CoFe 2 O 4 electrodes is observed, which is recognized as the Warburg impedance related to the diffusion and transport of counterions between the electrolyte (KOH) and the surface of the electrode material for the duration of the redox reaction. 28,52 3.4.4. Cyclic stability. The cyclic performance of the CoFe 2 O 4 electrode material prepared at RT was examined by GCD tests and is shown in Fig. 8. When the number of cycles was increased to 1500 cycles, a gradual increase in capacitance from 100% to 125% was observed. The increasing capacitance may be attributed to the activation of the electrode material with an increasing number of charge-discharge cycles. 53,54 The activation of the electrode material enhances the participating electroactive surface area. 8,55 Another possible explanation for the increasing capacitance might be structural changes within the CoFe 2 O 4 lms. Pseudocapacitive materials are known to undergo signicant structural or microstructural changes during chargedischarge processes. 56 A similar increase in percentage retention

Conclusions
Cobalt ferrite thin lms prepared by the pulsed laser deposition technique were used as electrodes for supercapacitor applications. The material formation was conrmed by XRD and Raman spectroscopy results. A specic capacitance of 777.4 F g À1 at 0.5 mA cm À2 current density was observed for the lms prepared at RT, which is three times higher than that for the lms prepared at 450 C. This is attributed to the rough and porous surface of the lms, which was conrmed by AFM. The lms prepared at room temperature exhibited high power density (3277 W kg À1 ) and energy density (17 W h kg À1 ) values. The electrode material showed improved capacitance with increasing cycles due to the increase in the electroactive area. Keeping in mind the reproducibility and adherence of lms prepared by PLD, the reported electrode materials could serve as a promising candidate for supercapacitor applications.

Conflicts of interest
The authors declare that they have no conict of interest.